In America, it is legal in quite a few states for people over the age of 18 to have sex with people who are at least 16. In florida, when you're 16 you can have sex with anyone up to the age of 24, so she and her boyfriend (if they are 15 and 21,) will pass that age criteria.

And yes I understand that there's a year difference between 15 and 16, but really how much different is it? It's perfectly legal.

They could be in an abstinent relationship up until the day she is 16 and it would still be legal. The main argument is on maturity, and if it's legal when you're 16 to have sex then it shouldn't be condemned to have a relationship up until that point, like it is now. Because there's no difference between 15 and 16.